1. Field of the Invention
The present invention relates to a high-efficiency optical parametric oscillator and, more particularly, to a tri-etalon optical parametric oscillator having a low reflectivity mirror set providing a low-finesse etalon effect for the pump, signal and idler beams which enhances the energy conversion efficiency.
2. Description of the Related Art
Optical parametric oscillators are well-known, non-linear optical devices capable of producing coherent radiation at a desired frequency via parametric amplification. In a conventional optical parametric oscillator (OPO), a pump source supplies a beam of laser light at a pump wavelength to an optical cavity bounded by end mirrors and containing a non-linear optical medium such as a non-linear optical crystal. As the pump beam propagates through the non-linear crystal within the cavity, photons at the pump wavelength are converted into photon pairs at two longer wavelengths, resulting in two lower-energy beams at these two wavelengths, conventionally denoted as the signal wavelength and the idler wavelength. The sum of the frequencies of the signal and idler beams equals the frequency of the pump beam. The particular wavelengths of the signal and idler beams are determined by a number of factors, including: the pump wavelength, the type and structure of the non-linear crystal, and the design of the optical cavity. By tuning the angle of the non-linear crystal, the energy can be selectively apportioned between the signal and idler beans.
Since typical operating conditions cause only a small fraction of the pump beam to be converted to the signal and idler beams in the initial pass through the non-linear optical crystal, the optical cavity of the OPO is generally designed to oscillate one or both of the parametrically generated beams such that the signal and/or idler beam is amplified in successive passes through the non-linear optical crystal. The OPO is considered a doubly resonant oscillator when both of the generated optical beams are resonated and is considered a singly resonant oscillator when only one of the generated optical beams is resonated. Specifically, the optical cavity can be designed with end mirrors which reflect only one of the signal and idler frequencies (singly resonant) or with end mirrors which reflect both the signal and idler frequencies (doubly resonant).
Normally, the signal beam, the idler beam, or both of the beams are resonated in the OPO cavity with reflectivity greater than 60% on one end mirror and greater than 99% on the other end mirror for the resonated wavelengths. Typically, for high power OPO cavities, the pump beam is not resonated due to damage limitations in the optical coating of the mirrors. To narrow the standard OPO device in wavelength, an additional element is usually incorporated into the cavity, or an external seed source is introduced into the cavity. The output is taken from the end mirror that has the lesser reflectivity, and the pump beam is either single passed or double passed through the cavity. As the fluence is increased, at some point the energy conversion process begins to operate in reverse, with signal and idler energy converting back to the pump wavelength. This undesirable back conversion self-limits the efficiency of the OPO device. Energy conversion efficiencies from the pump frequency to the signal or idler frequencies typically do not exceed a maximum of 35 to 40% in such conventional OPOs.
Moreover, OPOs are generally designed either for narrow linewidth or high energy, but not both. To extract a significant amount of energy from a standard OPO, the linewidth will broaden as the pump energy level is increased. Additionally, to achieve a high level of conversion efficiency, a standard OPO""s line narrowing elements cannot handle the intercavity fluence required without suffering damage. Seeding an OPO with a narrow linewidth diode laser can achieve modest conversion efficiency with a narrow linewidth, but the cost and complexity of this approach are great. Additionally, the diode laser has a limited tuning range of only a few nanometers, drastically restricting the signal and idler wavelengths achievable with the OPO device.
Accordingly, there remains a need for a tunable optical parametric oscillator having enhanced energy conversion efficiency, preferably to both the signal and idler frequencies, along with a spectrally narrow laser output and minimal beam divergence.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to efficiently convert energy in an optical parametric oscillator from a pump wavelength to at least one other wavelength.
A further object of the present invention is to develop an optical parametric oscillator having a low-finesse configuration for all three of the pump, signal and idler wavelength beams.
Yet a further object of the present invention is to design an optical parametric oscillator with a narrow spectral linewidth.
A still further object of the present invention is to provide an optical parametric oscillator with high energy.
Another object of the present invention is to achieve both a narrow linewidth and high energy output in an optical parametric oscillator.
Yet another object of the present invention is to construct an optical parametric oscillator which is low-resonant for the signal, idler and pump beams and behaves as an etalon for all three beams.
Still another object of the present invention is to design an optical parametric oscillator whose signal and idler outputs are tunable over a wide range of wavelengths.
It is a further object of the present invention to enhance energy conversion efficiency by increasing fluence in leading and trailing edges of the laser pulses via partial resonance, while limiting enhancement of the intra-cavity pump fluence during the peak of pulses to prevent excessive back conversion to the pump wavelength.
It is yet a further object of the present invention to compensate for walk off of optical beams caused by interaction with the non-linear medium within the optical cavity of an optical parametric oscillator.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
The optical parametric oscillator (OPO) of the present invention combines a partially-resonant mirror set (i.e., a low-finesse configuration for all three of the pump, signal and idler wavelength beams) to enhance the energy conversion efficiency, spectrally narrow the laser output, and decrease the output beam divergence by allowing a longer cavity length, thereby reducing the aspect ratio of the beam diameter to cavity length.
The low-finesse OPO includes an optical cavity bounded by two coupling mirrors, and a pump laser which supplies pump energy to the optical cavity at a pump wavelength. Preferably, the pump laser is a narrow-linewidth pump-laser source, such as an injection seeded Nd: YAG laser. A pair of walk-off compensated, non-linear optical crystals is disposed with the cavity and converts energy at the pump wavelength to energy at longer signal and idler wavelengths. The mirrors are configured such that each of the three beams (pump, signal and idler) is fed back into the cavity at a low percentage of approximately 10% to 30%. The result is a partial resonance of each of the beams, which creates an etalon effect that enhances the fluence in the leading and trailing edge of the pulse, thereby enhancing the energy conversion efficiency to the signal and idler wavelengths. During the peak of the pulse, more of the pump beam is converted to the signal and idler photons, and the enhancement of the intra-cavity pump fluence is reduced, thus preventing excessive back conversion of the two parametric wavelengths to the pump wavelength. The output signal energy emerges from one of the coupling mirrors, while the output idler energy emerges from the other coupling mirror.
Tuning of the OPO is limited only by the range over which the dichroic mirrors have uniform response (upwards of 100 nm or more), permitting a wide range of output wavelengths that can be generated. The linewidth of the output is self-narrowed, the exact value of which depends upon the values that make up the separate etalons at each wavelength. Due to the unique low-finesse, tri-etalon configuration of the OPO, the cavity length can be nearly doubled while still maintaining 90% to 95% of the output energy produced at a more conventional, shorter cavity length, giving rise to a higher beam quality (i.e., less beam divergence) without substantial loss of output energy.
In an exemplary embodiment, an input coupling mirror receives and transmits into the optical cavity between approximately 70% to 90% of the pump energy generated by the pump laser. The incident pump beam travels through the non-linear optical crystals, causing conversion of some pump energy to the signal and idler wavelengths. An output coupling mirror then reflects between approximately 70% to 100% of the pump energy back through the cavity and the non-linear crystals before 70% to 90% of the pump energy is transmitted out of the optical cavity through the input coupling mirror. The input coupling mirror reflects between approximately 70% to 100% of the energy at the signal wavelength and transmits between approximately 70% to 90% of the energy at the idler wavelength, while the output coupling mirror reflects between approximately 70% to 100% of the energy at the idler wavelength and transmits between approximately 70% to 90% of the energy at the signal wavelength.
The cavity length,is adjustable to maintain resonance of all three waves (i.e., pump, signal and idler). To maintain efficient energy conversion, a closed-loop servo system or the like evaluates the power of the output beam or a reduction of power in the rejected pump and adjusts the cavity length accordingly using a piezoelectric element or other known mechanism.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.